1. Field of the Invention
This invention relates to a method of annealing a semiconductor film by using a laser beam (hereinafter called “laser annealing”) and a laser irradiation apparatus (an apparatus including laser and an optical system for guiding a laser beam outputted from the laser to a treated object) for executing the method. The invention relates also to a fabrication method of a semiconductor device that includes a laser annealing step.
The term “semiconductor device” used in this specification designates all those devices that operate by utilizing semiconductor characteristics, and electro-optical apparatuses, light-emitting apparatuses, semiconductor circuits and electronic appliances are all semiconductor devices.
2. Description of the Related Art
A technology of forming thin film transistors (TFT) by using a semiconductor film (having thickness of several to hundreds of nm) formed on a substrate having an insulating surface has drawn an increasing attention in recent years. The thin film transistors have widely been applied to electronic devices such as IC and electro-optical apparatuses, and development as a switching device of an image display apparatus has been hastened, in particular.
A technology that applies laser annealing to a semiconductor film formed on an insulating substrate such as glass to crystallize the semiconductor film or to improve its crystallinity has extensively been studied. The semiconductor film uses silicon in most cases. In this specification, means for crystallizing the semiconductor film by using the laser beam and acquiring a semiconductor film having a crystalline structure will be referred to as “laser crystallization”.
In comparison with a synthetic quartz glass substrate that has widely been used in the past, a glass substrate is more economical, has higher workability and has the advantage that a substrate having a large area can be easily produced. This is the reason why the studies have been made. The reason why laser is preferably used for crystallization is because the melting point of the glass substrate is low. Laser can impart high energy to only the semiconductor film without much raising the temperature of the substrate. Through-put is by far higher than that of heating means using an electric heating furnace.
Because semiconductors having a crystalline structure contain a large number of crystal grains, they are called also “poly-crystalline semiconductor films”. The semiconductor film having the crystalline structure and formed by the application of laser annealing has high mobility. When a thin film transistor (TFT) is fabricated by use of this crystalline semiconductor film, TFT for a pixel portion and TFT for a driving circuit can be formed on one glass substrate.
When a laser beam is irradiated to the semiconductor film formed on the substrate in a scanning method of a laser beam such as laser crystallization, a laser beam shaped into a belt shape is irradiated in a longitudinal direction (Y direction) of the substrate, and the irradiated region is moved in a transverse direction (X direction) relative to the substrate so as to scan the laser beam. Laser irradiation apparatuses according to the prior art mostly employ a system in which a stage supporting thereon a substrate is moved in the X direction (or the Y direction) relative to a fixed laser beam region to scan the laser beam.
When the size of the substrate becomes great in the laser irradiation apparatus for moving the stage having the substrate supported thereon in the X and Y directions, however, a footprint (an area on a plane necessary for processing) becomes drastically great in proportion to the increase of the substrate size, and the apparatus becomes very great in size as a whole.
Sizes of 590 mm×670 mm, 600 mm×720 mm and 650 mm×830 mm have been used at present on production lines as the size of large glass substrates, and sizes of 680 mm×880 mm, 730 mm×920 mm or sizes exceeding them will be presumably used in future.
When a spot beam is used in the laser irradiation apparatus that moves the stage in the X and Y directions, at least a space corresponding to two substrate sizes in the X direction and a space corresponding to two substrate sizes in the Y direction are necessary as the area for moving plane-wise the substrate. When a linear beam is used, the space can be somewhat limited and yet a wide space is still necessary.
When the laser irradiation apparatus becomes very great in size, the moving speed is limited, as well. Particularly when solid laser of continuous oscillation (CW: continuous-wave) is used, a large burden will be applied to the laser irradiation apparatus if the stage (a stage supporting thereon the substrate) is moved at a moving speed of 0.5 to 2,000 cm/sec such as 100 cm/sec, for example.
On the other hand, another irradiation system fixes a substrate position, irradiates a laser beam by use of a galvanomirror or a polygon mirror and moves the irradiated region in a transverse direction (X direction) relative to the substrate to scan the laser beam. When the galvanomirror or the polygon mirror is used for scanning the laser beam, the increase of the footprint can be restricted even when the substrate becomes large in size. Therefore, this system has the advantage that the increase of the size of the overall apparatus can be avoided. In addition, because the laser beam is scanned by oscillation of a mirror that is light in weight, the moving speed of the irradiated region can be freely set and a large burden is not applied to the laser irradiation apparatus.
When the laser beam is scanned by use of the galvanomirror or the polygon mirror, however, an angle of incidence of the laser beam to the semiconductor film varies depending on the position. When the angle of incidence to the semiconductor film varies, a reflection factor of the laser beam on the substrate surface and an absorption factor of the semiconductor film to the laser beam vary, too. In consequence, the problem arises that uniformity of laser annealing treatment is low.